The present invention relates to lift cranes, and more particularly to rotatable connection systems for sectional boom members for cranes and the like.
Large capacity lift cranes typically have elongate load supporting boom structures comprised of sectional boom members secured in end-to-end abutting relationship. Predominantly, each of the sectional boom members is made of a plurality of generally disposed lacing or lattice elements. The terminal end portions of each chord are generally provided with connectors of one form or another to secure abutting boom segments together and to carry compressive loads between abutting chords. Typical connectors comprise male and female lugs secured by a pin carrying compressive loads in double shear.
An example 220 foot boom may be made of a 40 foot boom butt pivotally mounted to the crane upper works, a 30 foot boom top equipped with sheaves and rigging for lifting and supporting loads, with five sectional boom members in between: one 10 feet in length, one 20 feet in length and three 40 feet in length. Such an example boom has six boom section connections. Typically each section has four chords, and hence four connectors, making a total of 24 connectors that must be aligned and pinned to assemble the boom.
Large capacity cranes require very large boom cross sections. As a result, even when the boom segments are laying flat on the ground, the pin connectors between the top chords are typically eight feet or higher off the ground. The rigging personnel must either move a step ladder to each pin location or stand and walk along the top of the boom to reach the top connectors.
A 40 foot long sectional boom member may weight over 5,000 lbs. Thus, an assist crane is required to lift the boom member. One rigger usually then holds the suspended boom section in general alignment while a second rigger uses a large hammer (10 or 15 lbs.) to manually drive the pin, which typically has a long taper, into position. In the prior art, the pins connecting the boom sections are generally used to carry the compressive loads between chords. As a result, the pins have a tight fit, further increasing the difficulty in assembling the boom. As such, it may take three men (a crane operator and two riggers) four or more hours to assemble the example 220 foot boom. Where the crane is moved frequently, the costs to assemble and disassemble the boom may exceed the cost to lift and position the load for which the crane is used.
Efforts have been made to design sectional boom members with quick-connect systems. For example, U.S. Pat. No. 3,511,388 discloses a pin connection system for boom structures having tubular chord members. Tapered male lug members are disclosed for insertion, presumably with some rapidity, into female sockets. The lugs are then held together by a pin. Compressive loads are carried by machined surfaces on the perimeter of the lugs, slightly larger in width than thickness of the walls of the tubular members.
U.S. Pat. No. 5,082,128 discloses a quick-connect system where the connectors on the top chords have hook-like male lugs and female lugs with spaced members capturing a horizontal pin between them. FIGS. 10a-10c show how the hook-shaped member can be fit in place while the boom sections are not parallel, with a rotary motion (about the axis of the pins) bringing the boom sections into parallel alignment and mating up bearing surfaces on the end of each male lug with the inner face of each female lug. The horizontal neutral axis of the top chords (which appear to be tubular in cross-section) intersect the centerline of the pin, but does not intersect the compressive load bearing surface, nor is the compressive load bearing surface symmetrical about the horizontal neutral axis.
It would be preferable if compressive load bearing surfaces on connectors were symmetrical about the horizontal and vertical neutral axes of the chords to which they were attached. This would allow compressive loads to be transmitted through the connectors without creating bending moments in the chords. Also, chords having a right angle cross-section are frequently used on boom sections, and quick-connect systems for such chords would be useful.
U.S. Pat. No. 5,199,586 discloses quick-connect sectional boom members that have compressive load bearing surfaces that are not only symmetrical about the vertical and horizontal neutral axes of the chords to which they are attached, but are intersected by a line that is formed by the intersection of these neutral axes.
While the design of the connector of FIGS. 16-18 of the '586 patent have met with commercial success, and allow quick boom assembly when the boom is being constructed in a horizontal fashion, there are times when boom construction would be better carried out in a vertical fashion. For example, when there are job site space constraints, it is not always possible to construct a long main boom and a luffing jib boom on the ground in a layout position. Under these conditions, it is desirable to construct the main boom and just the luffing jib boom butt and luffing jib struts. These components are then boomed up until the luffing jib boom butt hangs vertical. It would be desirable if the next section of luffing jib boom could be brought in and connected while the connection points are as close to the ground as possible. To achieve this, the next section of boom should be oriented horizontally, and the top chord connection made. To do this, it is necessary to have a connection system that will then allow the boom section to rotate 90.degree. about the top chord section while the luffing jib is further raised and the new section of boom is allowed to swing under the luffing jib boom butt into vertical alignment. Thus, an easy, quick-connect system for boom sections that allows for top chord connections that can rotate through 90.degree. would be a great improvement.